The present invention relates to an LED lamp and more particularly relates to a white LED lamp that can be used as general illumination.
A light emitting diode (LED) is a semiconductor device that can radiate an emission in a bright color with high efficiency even though its size is small. The emission of an LED has an excellent monochromatic peak. To obtain white light from LEDs, a conventional LED lamp arranges red, green and blue LEDs close to each other and gets the light rays in those three different colors diffused and mixed together. An LED lamp of this type, however, easily produces color unevenness because the LED of each color has an excellent monochromatic peak. That is to say, unless the light rays emitted from the respective LEDs are mixed together uniformly, color unevenness will be produced inevitably in the resultant white light. Thus, to overcome such a color unevenness problem, an LED lamp for obtaining white light by combining a blue LED and a yellow phosphor was developed (see Japanese Patent Application Laid-Open Publication No. 10-242513 and Japanese Patent No. 2998696, for example).
According to the technique disclosed in Japanese Patent Application Laid-Open Publication No. 10-242513, white light is obtained by combining together the emission of a blue LED and the yellow emission of a yellow phosphor, which is produced when excited by the emission of the blue LED. That is to say, the white light can be obtained by using just one type of LEDs. Accordingly, the color unevenness problem, which arises when white light is produced by arranging multiple types of LEDs close together, is avoidable.
But the luminous flux of a single LED is too low. Accordingly, to obtain a luminous flux comparable to that of an incandescent lamp, a fluorescent lamp or any other general illumination used extensively today, an LED lamp preferably includes a plurality of LEDs that are arranged as an array. LED lamps of that type are disclosed in Japanese Patent Application Laid-Open Publications No. 2003-59332 and No. 2003-124528. A relevant prior art is also disclosed in Japanese Patent Application No. 2002-324313.
However, an LED lamp, which can overcome the color unevenness problem of the bullet-shaped LED lamp disclosed in Japanese Patent No. 2998696, is disclosed in Japanese Patent Application No. 2002-324313. Hereinafter, this LED lamp that can overcome the color unevenness problem will be described.
The LED lamp with the bullet-shaped appearance as disclosed in Japanese Patent No. 2998696 has a configuration such as that illustrated in FIG. 1. As shown in FIG. 1, the bullet-shaped LED lamp 200 includes an LED chip 121, a bullet-shaped transparent housing 127 to cover the LED chip 121, and leads 122a and 122b to supply current to the LED chip 121. A cup reflector 123 for reflecting the emission of the LED chip 121 in the direction indicated by the arrow D is provided for the mount portion of the lead 122b on which the LED chip 121 is mounted. The LED chip 121 is encapsulated with a first resin portion 124, in which a phosphor 126 is dispersed and which is further encapsulated with a second resin portion. If the LED chip 121 emits a blue light ray, the phosphor 126 is excited by the blue light ray to produce a yellow light ray. As a result, the blue and yellow light rays are mixed together to produce white light.
However, the first resin portion 124 is formed by filling the cup reflector 123 with a resin to encapsulate the LED chip 121 and then curing the resin. For that reason, the first resin portion 124 easily has a rugged upper surface as shown in FIG. 2 on a larger scale. Then, the thickness of the resin including the phosphor 126 loses its uniformity, thus making non-uniform the amounts of the phosphor 126 present along the optical paths E and F of multiple light rays going out of the LED chip 121 through the first resin portion 124. As a result, the unwanted color unevenness is produced.
To overcome such a problem, the LED lamp disclosed in Japanese Patent Application No. 2002-324313 is designed such that the reflective surface of a light reflecting member (i.e., a reflector) is spaced apart from the side surface of a resin portion in which a phosphor is dispersed. FIGS. 3(a) and 3(b) are respectively a side cross-sectional view and a top view illustrating an LED lamp as disclosed in Japanese Patent Application No. 2002-324313. In the LED lamp 300 shown in FIGS. 3(a) and 3(b), an LED chip 112 mounted on a substrate 111 is covered with a resin portion 113 in which a phosphor is dispersed. A reflector 151 with a reflective surface 151a is bonded to the substrate 111 such that the reflective surface 151a of the reflector 151 is spaced apart from the side surface of the resin portion 113.
Since the side surface of the resin portion 113 is spaced apart from the reflective surface 151a of the reflector 151, the shape of the resin portion 113 can be freely designed without being restricted by the shape of the reflective surface 151a of the reflector 151. As a result, the color unevenness can be reduced significantly.
By arranging a plurality of LED lamps having the structure shown in FIG. 3 in matrix, an LED array such as that shown in FIG. 4 is obtained. In the LED lamp 300 shown in FIG. 4, the resin portions 113, each covering its associated LED chip 112, are arranged in columns and rows on the substrate 111, and a reflector 151, having a plurality of reflective surfaces 151a for the respective resin portions 113, is bonded onto the substrate 111.
In such an arrangement, the luminous fluxes of a plurality of LEDs can be combined together. Thus, a luminous flux, comparable to that of an incandescent lamp, a fluorescent lamp or any other general illumination source that is used extensively today, can be obtained easily.
In fabricating the LED lamp 300 shown in FIG. 4, after the LED chips 112 have been mounted in columns and rows on the substrate 111, all of their resin portions 113 are preferably made at a time so as to cover the respective LED chips 112. Ideally, every LED chip 112 should be located at or around the center of the resin portion 113 as shown in FIG. 5. Actually, however, if the manufacturing process has significant tolerance, then not every LED chip 112 will be located at the center of its associated resin portion 113 to cause misalignment. As a result, some LED chips 112 may be exposed on the resin portions 113 as shown in FIG. 6. The LED chips 112 are likely to be exposed as shown in FIG. 6 particularly in the outer region of the matrix.
FIG. 7 illustrates LED chips 112a and 112b, which are located in the outer region and exposed on the resin portions 113. Specifically, the LED chip 112b is located on the outermost region, while the LED chip 112a is located on the second outermost region.
As shown in FIG. 7, there is no resin portion 113 on a part of the LED chip 112b facing the outermost region. Thus, (blue) light ray A emitted from the LED chip 112b is not mixed with the emission of the phosphor but is reflected by the reflector (not shown) to be output as it is (i.e., as a blue ray) in the direction pointed by the arrow G. A light ray (i.e., the blue ray in this case) emitted from an LED chip has directivity and does not mix with other chromatic rays easily. As a result, color unevenness is produced to make the white light emitted from a white LED lamp look as if the white light included blue components. The white LED lamp with such color unevenness is a defective product. Thus, such color unevenness decreases the yield and eventually increases the cost of white LED lamps.
Also, in the arrangement shown in FIG. 7, another blue ray A is radiated from the LED chip 112a, which is located next to the LED chip 112b. However, the light ray A emitted from the LED chip 112a is less noticeable than the light ray A emitted from the outermost LED chip 112b. This is because the light ray A emitted from the LED chip 112a mixes with a light ray B that has passed through the phosphor in the resin portion 113 covering the outermost LED chip 112a (e.g., a light ray containing relatively a lot of yellow components). As a result, the light as pointed by the arrow H looks more like white.
Consequently, in such a white LED lamp in which a plurality of LED chips are arranged, the blue light ray A emitted from the outermost LED chip 112b is a major factor of the color unevenness. In the example illustrated in FIG. 7, the LED chips 112 are fully exposed on their resin portions 113. However, even if those LED chips 112 are not exposed fully but are misaligned from their centers so much as to reduce the outermost thickness of the resin portions 113 significantly, the color unevenness problem is also caused by the blue light ray A.
In order to overcome the problems described above, a primary object of the present invention is to provide an LED lamp that produces light with significantly reduced color unevenness.